1. Yet Another Related Posts Plugin

Set which web page elements you want to take into account when calculating relevancy. By default the tool analyzes:

Titles;

Bodies;

Tags;

Categories:

Set the match threshold: the higher the match threshold, the more restrictive, and you get less related posts overall. The default match threshold is 5. If you want to find an appropriate match threshold, take a look at some post’s related posts display and their scores. You can see what kinds of related posts are being picked up and with what kind of match scores, and determine an appropriate threshold for your site.

And of course you can customize the look and feel of the related posts block by:

Setting the list type;

Creating the intro;

Showing / hiding excerpts;

Setting the number of posts to display, etc.

2. Arkayne

Arkayne is another WordPress plugin that can be used to interlink your older posts. Arkayne allows to link to related posts on your own blog as well as on other blogs in your chosen trusted circle (it can thus be a powerful networking tool).

According to their own words, Arkayne uses an advanced algorithm to define relevancy:

Instead of relying on tags, Arkayne automatically examines every visible word on a given post. After examining every post on your site Arkayne compares every post to the others to determine how contextually relevant they are… Please note Arkayne ignores all JavaScript content, and meta tags are not included in the contextual comparison

The links settings can be customized by going to your online Arkayne account and selecting the options there “Plugin settings” page:

Set the introductory header for the links (i.e. create an introductory phrase, like “Related posts “);

Add nofollow attribute to all external links pasted on your pages;

Add some additional URL parameters for tracking;

Set the number of cross- and innerlinks.

3. Google Related Links

Google Related Links is one of the most recent Google labs projects. It is executed as the javascript widget, so it can’t be crawled by search engine bots but the post relevancy is really high.